Crystal systems, xvii

Systems involving an interface are often metastable, that is, essentially in equilibrium in some aspects although in principle evolving slowly to a final state of global equilibrium. The solid-vapor interface is a good example of this. We can have adsorption equilibrium and calculate various thermodynamic quantities for the adsorption process yet the particles of a solid are unstable toward a drift to the final equilibrium condition of a single, perfect crystal. Much of Chapters IX and XVII are thus thermodynamic in content. 

An example of a tt-bonded complex is the remarkable cyanine oxonol system, 3-XVI 3-XVII, for which at least fourteen different polymorphs or solvates have been identified (Etter et al. 1984). Two of these, a gold and a red form (each containing a molecule of CHCI3 solvent per 1 1 complex, and hence true polymorphs) crystallize concomitantly and have been structurally characterized (Etter et al. 1984). Three of these polymorphs are shown in Fig. 3.2. Despite the fact that both of these dye molecules are known to be individually self aggregating (Cash 1981) the two... 

Another alkaloid with the oxazolidine system is samandaridine (XVII), C21H31NO3, also a secondary amine. Chemical reactions (9) and IR-spectra indicate that it possesses a five-membered lactone ring. Its structure was also elucidated by X-ray analysis. Samandaridine hydrobromide crystallizes from methanol in monoclinic prisms its space group is C2 with four molecules in the unit cell. The cell constants are a = 14.50 A, b = 6.15 A, c = 22.52 A, = 94° (10). 

Amphiphilic molecules have two parts, one hydrophilic and one hydrophobic, and form liquid crystals the textures of which are of great interest, since they are very close in morphology to biological materials and, in particular, cell membranes [17-19], showing a polymorphism related to that known in water-lipid systems (see Chaps. XV to XVII of Vol. 2 of this Handbook). The lamellar structure displays some usual defects and textures (Fig. 5). The bilayers are more or less separated by water (Fig. 5 a, b) this... 

Nonsteady behavior of electrochemical systems was observed by Fechner as early as 1828 [ii]. Periodic or chaotic changes of electrode potential under gal-vanostatic or open-circuit conditions and similar variation of current under potentiostatic conditions have been the subject of numerous studies [iii,iv]. The electrochemical systems, for which interesting dynamic behavior has been reported include anodic or open-circuit dissolution of metals [v-vii], electrooxidation of small organic molecules [viii-xiv] or hydrogen, reduction of anions [xv, xvi] etc. [ii]. Much effort regarding the theoretical description and mathematical modeling of these complex phenomena has been made [xvii-xix]. Especially studies that used combined techniques, such as radiotracer (-> tracer methods) ig. 1) [x], electrochemical quartz crystal microbalance (Fig. 2) [vii,xi], probe beam deflection [xiii], surface plasmon resonance [xvi] surface stress [xiv] etc. have contributed considerably to the elucidation of the role of chemisorbed species ( chemisorption), surface reconstruction as well as transport phenomena in the mechanism of oscillations. 

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